Are you using Ficoll-Paque? Or Histopaque or Lymphoprep? These are all trade names for the gradient material used to isolate PBMC. There is also just Ficoll - a sucrose polymer- trademark of Pharmacia. Sorry to ask such a basic point but assuming things leaves a lot out.

If you are trying to isolate mouse DC there are other methods rather than using peripheral blood.

You may want to check what cell types you have in the initial prep. Human PBMC contain 10-15% monocytes but mouse WBC are a different range of cell types. Also, the density of mouse lymphs is bit higher than human so if you use the most common formulation of Ficoll-Paque you will lose some cells in the pellet.

More details please and you'll get more specific input. Hope we can help.